The invention relates to voltage regulators and, in particular, to three-terminal voltage regulators. These devices respond to an unregulated input voltage and provide an output voltage that does not vary significantly in response to load variations or to input voltage variations. The devices also employ circuits that provide a substantially constant output voltage over a wide temperature range.
It is well known that voltage regulators have the best dynamio stability when their outputs are taken from the emitter of the power transistor. For example, the industry standard, LM117 series and the LM140 series devices, are relatively stable without external components. Conversely, when the output is taken from the collector of the power transistor, as is the case for the industry standard LM120 series and the LM137 series devices, a relatively large capacitor must be connected to the output terminal if stability is desired. The LM120 and LM137 specifications call for an output capacitor of at least one microfarad if tantalum and 10-25 microfarads if aluminum. Higher values are preferred.
While the above-mentioned devices are all of bipolar transistor construction, the same considerations apply to metal oxide semiconductor (MOS) construction. In particular, useful voltage regulators are being constructed using complementary MOS (CMOS) devices. In CMOS the above remarks apply to the sources and drains of the power transistors. When the source of the power transistor provides the output the circuits are relatively stable. However, when the output is taken from the power transistor drain a large output capacitor must be employed.
The reason for the above-expressed instability is understood to be due to the feedback loop gain. In a voltage regulator the power transistor is a part of a high gain negative feedback loop that is referenced to a constant voltage. When the power transistor emitter/source electrode provides the output its voltage gain is less than unity and the circuit tends to be stable. When the output is taken from the collector/drain the voltage gain depends upon the load impedance and can be substantial. A large output capacitor is thus required for limiting the a-c gain so that stability is achieved.
In the following discussions bipolar transistor emitters and MOS transistor sources are referred to as the low impedance electrodes. The bipolar transistor collectors and MOS transistor drains are referred to as the high impedance electrodes These characterizations provide the functional device equivalents. The bipolar transistor bases and MOS transistor gates are referred to as control electrodes because they are also functionally equivalent.
Another power supply characteristic is its dropout voltage. This is defined as the input-output voltage differential at which the circuit ceases to regulate against further reductions in input voltage. As a practical matter, low dropout voltage is a virtue and is regarded as important in battery operated applications. Typically, the dropout voltage is on the order of 2 volts for the above-referenced devices and is inversely related to temperature. All of the above-designated device families employ a Darlington connected power output or pass transistor. This means that the Darlington input transistor base must be at least two x V.sub.BE above the emitter and the collectors must be at least a V.sub.SAT above this. However, the LM120 needs V.sub.BE + V.sub.SAT. At the lower operating temperatures this is typically a voltage drop of about 2 volts. This voltage drop is sometimes called `headroom` because the voltage regulator input must be high enough so that it will accommodate the output voltage plus the dropout voltage.
Examples of low dropout regulators are the LM2930 and LM2931 series devices. These are respectively rated at 150ma and 100ma and both have a dropout rating of less than 0.6 volt at rated current. Because their outputs are taken at the collector of a PNP transistor, they both require capacitors at their output terminals. The minimum capacitor values are specified at 10 and 22 microfarads respectively.